A carbon fiber is superior in specific strength and specific elastic modulus than other fibers and is industrially used widely as a reinforcing fiber or the like combined with resin by taking advantage of its lightweight characteristics and excellent mechanical characteristics.
Conventionally, the carbon fiber is manufactured in the following manner. First, a precursor fiber is subject to a pre-oxidation treatment by heating the precursor fiber in heated air at 230 to 260° C. for 30 to 100 minutes. This pre-oxidation treatment causes a cyclization reaction of the acrylic fiber, increases the oxygen binding amount, and produces a pre-oxidation fiber. This pre-oxidation fiber is carbonized, for example, under a nitrogen atmosphere, with use of a firing furnace at 300 to 800° C., and under a temperature gradient (first carbonization treatment). Subsequently, the pre-oxidation fiber is further carbonized under a nitrogen atmosphere, with use of a firing furnace at 800 to 2100° C., and under a temperature gradient (second carbonization treatment). In this manner, the carbon fiber is manufactured by heating the pre-oxidation fiber from an external portion thereof in the heated firing furnace.
In a case of manufacturing the carbon fiber in the above manner, the temperature must be raised gradually over time to avoid insufficient carbonization of an internal portion of the fiber to be carbonized. The firing furnace heating the pre-oxidation fiber from the external portion thereof has a low heat efficiency since the furnace body and the firing environment as well as the fiber to be carbonized are also heated in the firing furnace.
In recent years, manufacturing the carbon fiber by irradiating the fiber to be carbonized with microwaves and thereby heating the fiber is attempted. In heating a substance by means of the microwaves, the substance is heated from the internal portion thereof. Thus, in the case of heating the fiber to be carbonized with use of the microwaves, the internal portion and the external portion of the fiber can be carbonized uniformly, and reduction of manufacturing time for the carbon fiber is expected. In the case of heating the fiber with use of the microwaves, a target to be heated is only the fiber to be carbonized, and a high heat efficiency is thus expected.
Conventionally, as methods for manufacturing a carbon fiber with use of microwaves, methods in Patent Literature 1 to 4 are known. These methods have limitations such as providing a decompression unit for microwave-assisted plasma, adding an electromagnetic wave absorber or the like to a fiber to be carbonized, performing preliminary carbonization prior to heating by means of microwaves, requiring auxiliary heating, and requiring multiple magnetrons and are not suitable for industrial production.
Further, since the carbon fiber has a high radiation coefficient on its surface, it is difficult to sufficiently raise the firing temperature at the time of irradiating the fiber to be carbonized with microwaves and thereby carbonizing the fiber. Thus, in a case of manufacturing the carbon fiber only with irradiation with microwaves, a carbon fiber having a high carbon content rate cannot be obtained.